Electron discharge device having a stand-by cathode



Dec. 29, 1964 s. H. BORMAN L 3,163,793

ELECTRON DISCHARGE DEVICE HAVING A STAND-BY cATHoDE Filed March 19, 1962 Q E Y L la .l I

/20 i Naz fdp/V j? United States Patent O ELECTRN DISCHARGE DEVICE HAVING A STANi-BY CATHGDE Samuel H. Berman, Willow Grove, Fa., assigner to Philco Corporation, Philadelphia, Pa., a corporation of Delaware FiledlViar. 19, 1962, Ser. No. 189,725 Claims. (Ci. 313-71) This invention relates to electron-discharge devicesand more particularly to an improved method and means for extending the emission life of such devices.

The term electron discharge device as used in the specication and claims is a generic classification of vacuum-enclosed devices inclusive of cathode ray tubes, radio receiving tubes, television receiving tubes, gas discharge tubes and the like.

The invention while having broad application to electron discharge devices generally will be described in specific relation to its use with cathode ray tubes.

The useful life of electron-discharge devices is in large degree determined by the emission life of the cath-l ode.

Prior art approaches directed at improving this aspect of performance have largely taken the form of using standby cathode heaters, or of reactivating the cathodes electron-emissive surface by use of a booster. This latter approach acts to raise the heater current thermionically boiling oft additional electrons'. These techniques, however, at best have proved but partially successful and have produced only modest extension of the tubes emission life.

Accordingly, it is an object of this invention to provide a method of and means for improving the emission life of electron discharge devices which overcomes the limitations of the prior art.

Another and more particularized object of the invention is to provide a unique method and means for extending the emission life of a cathode ray tube on failure of its cathodic element.

These and other objects Within contemplation will be more readily understood by reference to the following detailed description and drawing in which:

FIGURE 1 is a partially cut away elevational view of a cathode ray tube electron gun assemblage embodying structure of the present invention;

FIGURE 2 depicts -another type of electron gun assembly embodying :an alternative structural form of the present invention; p

FIGURE 3 is an exploded view of the cathode-grid assembly shown in FIGURE 2 illustrating details of its structural organization; and Y FIGURE 4 is a schematic Wiring diagram depicting one form of apparatus designed to practice the method of this invention.

Briey described, the invention comprises incorporating within :an electron discharge device an additional reserve or standby cathode.

The reserve cathode is maintained in an inert or unactivated state until failure of the devices original cathode. Failure may, for example, be said to have occurred when emisison decay of the devices original cathode has progressed to a point such that the cathode requires replacement. At this point the spent cathode is deenergized and the standby cathode is activated and brought into operation by suitable circuit connection or modification and by application to the tube of the requisite cathode activating procedure.

The essence of the invention resides in the discovery that the use of a standby or reserve cathode is only effectual in extending the emission life of the electron-discharge device if the standby cathode is maintained in an ice inert or unactivated state during the standby period, i.e. does not have its electron emissive surface converted from the usual double or triple carbonate or nitrate form to the oxide state, until the cathode is to be used as a replacement. Simply to use an extra cathode Which has had its originally non-emissive cmbonate or nitrate coating converted to the oxide form, or emissive state, from the time of the tubes initial fabrication has proven ineifectual. It has been found that while the oxide coating is very susceptible to poisoning by gas-ion bombardment, the air-stable carbonate or nitrate form is not. Experimentation has revealed that bombardment of the unactivated coating prior to conversion has little detrimental elect on the cathodes ultimate emission life. On the other hand, gas bombardment of the coating after its conversion, even though the cathode is not used as a source of electrons severely attenuates the cathodes useful life.

In one of its apparatus forms the invention is featured by the provision of a dual electron gun structure each gun incorporating its own separate cathode element. This construction is shown in FIGURE 1 -and comprises two separately functioning electron guns 10 and 12, arranged paraxially of the cathode ray tubes longitudinal axis. Only the neck portion 14 of the tube is shown, the glass in the foreground having been broken away to provide an unobstructed View of the compound gun structure.

These structures depict an electron gun of the type typically used in electromagnetically focussed cathode ray tubes, each individually Icomprising a longitudinally oriented cathode element 16, control grid 18, commonly referred to as G1, screen grid 20, referred to `as G2, and the anode element 22er G3. Surmounting this composite assembly is a getter retaining ring 24 mounted on a supporting rod 26 secured to the outwardly extending peripheral flange 28 of anode 22. The ring 24 is of generally channel-shaped cross sectional configuration for storage of the getter material, the arrangement being such as to permit external inductive ashing of the ring-con- The oxide-coated cathode 16 is of cylindrical construction and conventionally consists of a metal base, of nickel, konel, or platinum alloys, the upper end of which is closed and coated with a layer of rare-earth oxides, usually barium and strontium or calcium, barium and strontium. The cathode chosen for illustration and shown in FIGURE l is approximately 1A in height, and has an OD. of .123" and anLD. of .105. Since these oxides are unstable in air, the cathodes are first coated With a fluid suspension of barium and strontium or of barium,

strontium and calcium carbonatos or nitrates by spraying` or dipping. The cathode is centrally located within the bore of the control grid 1S by means of an insulative ceramic sleeve 34. The upper surface of the cathode is axially spaced from the control grid 18 by means of an eyelet-type metallic spacer 36. The unit when Iassembled is locked in place by a retaining ring 38 wedged into lockling position within the open end of grid G1. Coiled within the cavity of the cathode cylinder is a helically Wound heater element 40 comprised of a ne, highresistance Wire, such as tungsten, sheathed in an electrically insulative coating of refractory oxide, such for example, as aluminum oxide. The constructionalfeatures of the two electron gun assemblies are identical.

` The preferred mode of installing the assemblage within the neck portion 14 of the cathode ray tube is lirst to secure the assembled gun structures to the stem or header 42.. This is principally accomplished through securement of the pin extensions 46 and 4S, as by spot welding, to wall portions of 'the G1 electrodes. To provide for evacuation of the tube the stem is provided with exhaust tubulation l). At this stage of assembly the unit is positioned within the tube neck and after further tube processing the periphery 52 of header 42 is hermetically sealed to tube Wall portions 54.

In accordance with the method teachings of this invention and following out gassing of the gun elements only one of the electron guns is readied for operation through activation of its cathode, the cathode associated with the standby gun being left in a dormant, unactivated state. However, the standby cathode desirably is degassed prior to activation of the other cathode by applying a low voltage to the standby cathodes ilament sufficient for this purpose but not high enough lto initiate carbonate conversion.

This may be accomplished, for example, by applying 7 vollts to the heater for about tive minutes. In the illustrated embodiment the coating of the unactivated cathode consists of barium and strontium carbonate.

Cathode activation consists of thermally decomposing the cathodes coating of metal carbonates to the oxide form and may be accomplished by impressing on the cathode element a timed schedule of voltages suicient to heat the cathode to a temperature capable of eiecting carbonate conversion but not such'as to cause disruption or peeling of the coating. A preferred arrangement is to effect conversion ofthe cathodes coating duringthe exhaust phase of tube manufacture. One illustrative conversion schedule designed to effect conversion of the double carbonate coating carried by cathodef16 is outlined in Table I below, conversion ofthe cathode taking place just prior to tip off, as a result of the 9-11-13 volt sequence of exposures. The gases produced by decompo-V sition are removed during tube evacuation. In the activadion process some of the oxide is reduced to the pure metal, which then forms an adsorbed surface layer as Well as increasing the conductivity of the oxide layer by i dispersal therethrough. This layer is electropositive to the base and-reduces the work vfunction by setting up a field to aid electron flow.

Following tip-off, getter material contained within ring. 24, such, for example, as aluminum-stabilized barium, is inductively heatedtoabOut 850 C. causing barium diffusion land dispersal to the tube walls. The barium in this highly dispersed state presents an extensive surface area having a relatively high sorptive capacity for the gases normally encountered in large quantities in vacuum tubes: i.e. O2, CO, CO2, N2, H2, and H2O. During getter ashing the cathode heater is desirably kept energized. One activating procedure is to impress on the cathode heater during this phase of tube fabrication 6 to 7 volts for a period of about l5 minutes to insureoptimum outgassin'giof the cathode and maximum adsorption by the getter of any gaseous decomposition products.

Tube fabrication is completed by aging to achieve full Table Il G1 Voltage, Heater Time v. Voltage, v.

3 min 5. o 3 mm 7.0 3 min-- 9. 0 2 mm 11` 0 1 mm 12.0 so min +10 8.0

During operation of the one gun, the cathode coating of the reserve gun is lett in the unactive or inert carbonate form. By retaining the standby cathode in this condition its life expectancy is not perceptibly shortened. If, on. Vthe other hand, the standby cathode is activated at the same time as the original cathode is placed in operation, its ultimate emission life is found to be Vappreciably reduced, if not totally destroyed. This result, as previously explained, has been found to occur notwithstanding the fact that the converted cathode has not been used as the electron generating element of the tube. Extensive experimentation indicates that by using a standby cathode, the coating of which is left unaotivated until the cathode is to be used as a replacement part, coupled with its ideal vacuum storage within the tube, results in a life expectancy. fully equivalent to that of the tubes original cathode.

Accordingly, when emission performance of the original cathode, here arbitrarily assumed to be the cathode associated with electron gun 10, becomes unsatisfactory or in fact if the cathode malfunctions for any reason, its heater is disconnected and the heater to the standby cathlode connected in circuit. Heater changeover may be affected by simply rewiring the circuit or by use of a socket adaptor interposed between the tube pins and the conventional CRT socket. When this changeover has been made one recommended procedure is inductively to iiash the auxiliary getter 56 provided for this purpose, and then to impress on the newly connected heater the sequence of voltages outlined in Table II. This timed schedule of voltages, although originally used to effect only aging of the cathode in initial tube fabrication, may be used both to convert the carbonate or nitrate coating of the standby cathode to the oxide form and fully to activate or age the cathode to optimize its emission characteristic. One circuit arrangement designed for carrying out the tabulated sequence of activating voltages is shown in FIGURE 4. Since the G1 grids are connected by an electrically conductive strap 56, terminal 59 of the actuation circuit may be connected either to pin 69 or to the pin, not shown, associated with lead 48. Terminal 61 of the activating circuit is connected to ground and terminals 62 and 64 are electrically connected through the abovementioned adaptor socket, or other suitable means, to heater filament pins 66 and 68, terminal 64 being attached to the ground connection of the heater filament. With fthe connections thus made the schedule of voltages outlined in Table II may be carried out by simply adjusting the rheostat '70 to get the Vdesired voltage which can be read on voltmeter 72. Since the voltage-time relationship is not critical the period during which each particular voltage is impressed on the heater may be measured by use of a stop watch or by simply observing the second hand on a standard wristwatch. Although the circuit may be considerably retined by incorpora-tion of a suitable timercontrolled stepping device whereby automatically to achieve the desired voltage-time relationships, the suggested approach using the simplied circuit arrangement illustrated will produce fully satisfactory results.

Activation of cathode 16 is completed by closing switch 74 which in the circuit shown, with contact 59 connected to pin 60, places a positive ten volt bias on control grid 1S. To achieve peak emission performance, pursuant to the schedule outlined in Table II, ythis voltage is applied for approximately 30 minutes while maintaining the heater at an 8 volt potential. Once the coating of the standby cathode has been converted to the oxide form the cathode socket is reconnected to the modified circuit and the tube thereby readied for operation. Any misalignrnent of the electron beam resulting from changeover may be readily compensated for by adjustment of the beam-centering magnets commonly used in conventional television receivers. By employment of the method and means described, the total emission life of a cathode ray tube is appreciably extended and in many applications doubled.

An alternative structural form of the invention, as applied to an electrostatically focussed tube, is shown in FIGURE 2. Again, only the neck portion of the tube is shown for clarity of illustration, all other parts of the tube being of conventional construction. The electron gun comprises a three element electrostatic focussing assembly 72, screen grid 74 and a cathode-control grid module 76. The screen grid and focussing assembly are maintained in axial spaced relation by insulative beading 7S to which they are staked. In accordance with the invention the cathode-control grid module is provided with two separate, transversely disposed cathodes S1 and 82 each provided with its own individual heating element. The heaters are so connected that only one is initially operable. In this construction, the box-like control grid S0 and screen grid '74 serves as common electrodes for the individual cathode elements, each electrode being provided with two apertures located in direct overlying relation to their respective cathodes. In the embodiment shown these apertures are located on .060 inch spaced centers to accommodate the .060" between centerlines spacing of cathodes 8-1 and S2.

The focussing assembly 72 is of generally conventional construction with the exception that the parts are of slightly larger bore than standard gun parts to accommodate the somewhat greater beam widening resulting from the use of off-center cathodes.

The transversely disposed cathode elements 81 and 82 are initially coated with material capable of activation to an electron-emissive state which material in its initial state in non-electron emissive or inert and stable in air. Examples of such materials have been given previously and consists, for example, of barium, calcium, and strontium in both the carbonate and nitrate forms. Only one of the cathode elements, here assumed for purposes of explanation to be cathode 81, is initially activated at the time of the tubes original fabrication, cathode 82 being installed but left in an unactivated condition. These cathodes are arranged pai-axial to the tube axis and are housed within the cathode-control grid module 76. For simplicity of design each of the cathode sleeves is electrically joined, by means not shown, to the same external connection and each of the cathode heaters are provided with a common ground leg 84. To facilitate a clearer understanding of the interrelation of parts comprising the composite gridcathode assembly the exploded perspective view shown in FIGURE 3 has been employed.

This assembly comprises the insulative support members or plates 86, preferably made of mica or other suitable insulating material, disposed in parallel spaced relation and adapted to support, by means of suitably positioned apertures punched therein, the structure comprised of two cathode sleeves 81 and 82, the box-like control grid 80, and channel-shaped, electrically conductive side rails S8. To aid in installing the cathodes, the tubular sleeves are dimpled in regions S9 and 90, these prominences serving to provide a locating stop when inserting 6 the cathode sleeves into their respective supporting apertures 91 and 92. The cathodes are retained in this oriented position by placement of their opposite ends into cooperating apertures provided in the opposing support member 86 and by later spot welding a conductive strap, not shown, to those portions of the cathode sleeve which project beyond the louter surface of the forward support member 86. The cathodes when completely installed are suspended between the mica suppont members 86 in a plane transverse the electron beam axis of the cathode ray tube. The cathodes 81 and 82 are aligned with their respective grid apertures 93 and 94 and with their electron emissive surfaces 95 and 96 positioned in confronting relation thereto, it being understood that at this stage of assembly the surfaces of both cathodes ar still coated with ,the inert, carbonate or nitrate form of the coating. The

f term inert as used herein signifys a material which is not lthennionically electron-emissive when exposed to ternperatures to which cathodes are conventionally subjected in normal operation.

Embracing the cathode is control grid S0, this member being located in accurate spaced relation to the cathodes by positioning of the grid locating tabs 98 Within their respective mating apertures located in the support members 86. The control grid is desirably of box-like configuration to increase its structural rigidity.

Bounding the control grid 30, but electrically insulated therefrom, are the side rails 8S, these members preferably being made of electrically conductive material and supported by the members 36 in a manner similar to that described for the control grid S0. The side rails are designed to be assembled to the screen grid 74, as by spot welding.

The spacer plate 102 facilitates juncture of the cathodecontrol grid module to the screen grid electrode, in a manner hereinafter described, and also provides accurate, simplified means for varying the spacing between the screen and control grids to provide a convenient method of varying tube transconductance. Further details of this constructional feature may be obtained by reference to US. Patent 2,905,848, filed October 18, 1957 and assigned to the assignee of the present invention. This plate is congured 'to bridge the modular assembly in such manner that the screen grid 74 to which the plate 102 is desirably iirst secured, is supported from the upper flanges of the side rails 68. The axial spacing between these members is accurately determined by the thickness of the spacer plate 102.

On alignment of the screen grid and control grid apertures, the spacer plate 102, which has previously been afiixed to screen grid 74, is spot welded to upper flange surfaces of the side rails 8S, producing a composite unitized electrode assembly. The screen grid 74, along with its cathode-controll grid appendage 76 is then staked, along with the electrostatic focussing assembly 74, to insulating pins 103 for retention in the desired axial spaced relation.

The electron gun assembly is next mounted to the header and installed within the cathode ray tube neck portion 112. The installation procedure is substantially the same as previously described. Following installation, the electron gun parts are outgassed by inductive heating and the tube exhausted. Prior to tip oif the carbonate or nitrate coating carried by cathode S1 is converted to the oxide form by heating the cathode to a temperature between 950 C. and 1050 C., the onset of breakdown being observable above about 950 C. This conversion is desirably accomplished by passage of current through the heater filaments `associated with cathode S1. The decomposition products resulting from conversion are removed during tube pump out, followed by tip off of the exhaust tubulation 114. As in the previous example, the coating of the auxiliary or standby cathode, in this case cathode S2, is left in the carbonate or nitrate form until needed as a replacement for cathode 81.

After conversion of the carbonate'into oxide has been completed, full activation of cathode 81 isiachieved by raising its temperature somewhere in the neighborhood of 1100 C., the exact temperature depending on the type of coating used and the base metal employed. It should be understood that cathode conversion and activation, as such, and apart from the standby concepts of the present invention, are known procedures inthe prior art, the most desirable voltage sequence to be impressed upon the cathode heater for this purpose being generally empirically determined and being a function of the type and form of the particular cathode under consideration. A representative activation procedure for converting and aging an oxide cathode of the type shown in FIGURES 2 and 3 is set out in Table III below.

Table 111 Time Gl Voltage Heater Voltage In general aging is done after seal-off. A well-activated cathode will permit drawing of continuous current of 5 00 mai/cm.2 for many thousands `of hours.

When cathode 81 fails, as when its emission performance becomes unsatisfactory, its heater is disconnected and the heater to cathode 32 is wired into the filament circuit. As in the previous example, this may be accomplished by actually rewiring the circuit or'by use of an appropriate socket adapter which disconnects pin 116 leading to the heater of cathode 81, and establishes a connection 11S leading to the heater filament of cathode S2. When this change has been made the auxiliary getter 120 is current flashed by application of the proper voltage to pins provided for this purpose. The coating of cathode S2 is next converted and aged by application thereto of the requisite sequence of timed voltages. The circuit for accomplishing current ilashing of the getter and activation of the cathode coating has not been shown, the circuit being similar to that previously illustrated with the exception of v example by use of two separate electron gun assemblies or by the use of one electron gun modified to accommodate a pair of cathodes, and by initially activating only one of the cathodes, the standby cathode will keep .indefinitely within the tube vacuum until needed.

While preferred embodiment, illustrative of the apparatus and method aspects of the present invention have been depicted and described, modifications may be made therein without departing from the scope of the present invention. It will be understood, therefore, that such changes and modifications are contemplated as come within the purview of the appended claims.

I claim: Y

1. The method of extending the emission life of electron discharge devices of a type in which the electron beamgenerating and control elements, inclusive of a first cathode, are housed within an evacuated enclosure, which method comprises: incorporating Within said enclosure, during fabrication of the device, anv auxiliarycathodic element coated with inert material'activatable to an electron-emissive state; activating said material on failure of said rst cathode; and connecting the newly activated cathode as the electron-emitting source of said device in electrical substitution for the failed cathode.

2. The method of extending the emission life of electron-discharge devices of a type in which the electrongenerating and control elements, inclusive of a rst cathode, are housed in an evacuated enclosure, which method comprises: incorporating within said enclosure during fabrication of the device, a standby cathodic element coated with air-stable material `chemically convertible from a non-electron-emissive state to an electron-emissive state; maintaining said material in an unconverted state during the useful emission life of said first cathode; chemically converting said material, by thermal activation thereof, to an eiectron-emissive state on emission failure of said first cathode; and including said cathodic element in said electron-generating and control assemblage in electrical substitution for the iirst cathode.

3. The method of extending the emission life of an electron-discharge device of a type whose cathode is housed within an evacuated enclosure, which method comprises: incorporating within said enclosure .a second cathodic element coated with inactive material thermally activatable to an electron-emissive state; maintaining the coating of said second cathodic element in its inactive state during satisfactory performance of said cathode; thermally activating said second cathodic element to permit its electrical substitution for said cathode on its failure to perform satisfactorily.

4.The method of extending the emission life of electron-discharge devices of a type whose cathode is housed within an evacuated enclosure, which method comprises: incorporating within said enclosure, during abrication of the device, a second cathodic element and associated heater, which element is coated with one or more compounds selected fromV the groupconsisting of barium carbonate, strontium carbonate and calcium carbonate; activating the coating, on defective performance of said cathode, to the oxide state by application to said heater of a predetermined sequence of timed voltages; and connecting the newly activated cathodic element in operative relation within said device.

5. The method of modifying a cathode ray tube gun structure to permit extension of the tubes emission life, which comprises: incorporating within said gun structure, during fabrication thereof, an auxiliary cathode coated with inactive material capable of activation to an electronemissive state; and providing means facilitating activation of said material and for utilizing the auxiliary cathode as a substitutional electron-generating source within said` gun structure.

6. In an electron-discharge device, the combination comprising: a pair of electron gun assemblages the first of which contains an electron-ernissive cathode and the which is electron emissive and the second of which is,-

coated with inert material capable of later activation to an electron-eniissivc state; means to facilitate activation of said second cathode to an electron-emissive state; and meansY providing for utilization of the activated second cathode as the electron-emitting source of said gun in substitution for the otherof said cathodes.

8. The method of preparing a vacuum-enclosed electron-discharge device for extension of its operational life on failure of its cathodic element, which method comprises: incorporating within said device an auxiliary cathode coated with inactive material capable of being activated to an electron-emissive state; and providing means facilitating activation of said material, and for connecting the activated cathode as a substitution elecron generating source for the failed cathodic element.

9. The method of preparing a cathode ray tube for eX- tension of its emission lhce on failure of its original cathode which comprises: incorporating Within said enclosure a standby cathode coated with inactive material capable of being thermally activated toan electron-emissive state; and providing means facilitating thermal activation of said material and or utilizing the activated reserve cathode as a substitutional electron generating source for said tube.

10. The method oi' extending the emission life of a cathode ray tube, which comprises: incorporating Within said tube, in addition to the tubes normal cathode cornplement, auxiliar; cathodic means coated with materiai covertible to an electron-emissive state; converting said material to its electron-emissive state on cathode failure of said tube; and providing for'utilization of the newly converted cathodic means as the electron-generating element of said tube.

References Cited in the lite of this patent UN TED STATES PATENTS 

2. THE METHOD OF EXTENDING THE EMISSION LIFE OF ELECTRON-DISCHARGE DEVICES OF A TYPE IN WHICH THE ELECTRONGENERATING AND CONTROL ELEMENTS, INCLUSIVE OF A FIRST CATHODE, ARE HOUSED IN AN EVACUATED ENCLOSURE,WHICH METHOD COMPRISES: INCORPORATING WITHIN SAID ENCLOSURE DURING FABRICATION OF THE DEVICE, A STANDBY CATHODIC ELEMENT COATED WITH AIR-STABLE MATERIAL CHEMICALLY CONVERTIBLE FROM A NON-ELECTRON-EMISSIVE STATE TO AN ELECTRON-EMISSIVE STATE; MAINTAINING SAID MATERIAL IN AN UNCONVERTED STATE DURING THE USEFUL EMISSION LIFE OF SAID FIRST CATHODE: CHEMICALLY CONVERTING SAID MATERIAL, BY THERMAL ACTIVATION THEREOF, TO AN ELECTRON-EMISSIVE STATE ON EMISSION FAILURE OF SAID FIRST CATHODE; AND INCLUDING SAID CATHODIC ELEMENT IN SAID ELCTRON-GENERATING AND CONTROL ASSEMBLAGE IN ELECTRICAL SUBSTITUTION FOR THE FIRST CATHODE. 